In a radar apparatus used to obtain information on the presence, distance and speed of an object, this information is usually obtained from the presence of a signal reflected from the object which is obtained by emitting electromagnetic waves thereto, detecting the time elapsed from the transmission of the signal to the reception of reflected waves, and detecting the Doppler shift in the received signal due to the movement of the object being detected.
In an aircraft-detecting radar which determines the position of a flying aircraft, for instance, electromagnetic waves of a frequency of about 1 GHz are emitted from a parabolic antenna, and the presence of the aircraft and the distance thereto are determined from the presence of electromagnetic waves reflected from the aircraft and the time delay from the transmission to the reception of the reflected waves. If the speed of propagation of the electromagnetic waves is v and the time from the transmission of the electromagnetic waves to the reception of the reflected electromagnetic waves is t, the distance d from the radar apparatus to the aircraft can be expressed as follows: EQU 2d=vt (1)
The speed of propagation of the electromagnetic waves is given by the following equation which is obtained by solving the Maxwell's equation: ##EQU1## where .omega.: angular frequency of electromagnetic waves,
.epsilon.: permittivity of the medium, PA1 .mu.: permeability of the medium, and PA1 .sigma.: conductivity of the medium.
When the medium through which the electromagnetic waves are propagating is the atmosphere, as in the case of aircraft-detecting radar, the permittivity .epsilon. and the permeability .mu. have the same values as those in a vacuum: .epsilon.=8.85.times.10.sup.-12 F/m and .mu.=1.26.times.10.sup.-6 H/m. The conductivity .sigma. is virtually zero since the atmosphere can be regarded as an insulating substance. Consequently, the speed of propagation v of electromagnetic waves in the atmosphere is equal to the speed of light, i.e., 3.times.10.sup.8 m/sec. When the medium is the atmosphere, therefore, the distance to the object being detected can be obtained from just the time required for receiving the reflected waves.
It has recently become necessary to probe for petroleum and minerals under the ground to search for energy resources, or to probe the ground to investigate things buried underground. Electromagnetic waves can be propagated not only through the atmosphere but also through earth, and therefore an object can be detected from the level of a reflected signal even when a conventional radar apparatus using a single reception frequency is employed. Soil mainly consists of silica SiO.sub.2, alumina Al.sub.2 O.sub.3 and water H.sub.2 O, and it is known that the weight ratios of SiO.sub.2, Al.sub.2 O.sub.3 and H.sub.2 O in soil can be 60 to 85%, 10 to 30%, and 1 to 10%, respectively, depending on the nature of the soil.
The speed of propagation v of electromagnetic waves is determined by the permittivity .epsilon., permeability .mu. and conductivity .sigma. of the propagation medium, as shown by Equation (2), so that the values of .epsilon., .mu. and .sigma. in soil are determined by the substances making up that medium and the composition ratios thereof. In dry ground (SiO.sub.2 :85%, Al.sub.2 O.sub.3 :10%, H.sub.2 O: 1%), for instance, the permittivity .epsilon. is about four times that in a vacuum and the conductivity .sigma. is 10.sup.-4 to 10.sup.-5 S/m; while in damp ground (SiO.sub.2 :60%, Al.sub.2 O.sub.3 :30%, H.sub.2 O:10%, the value of .epsilon. is about eight to thirty times that in a vacuum and the conductivity is approximately 10.sup.-1 to 10.sup.-3 S/m. Although the permeability .mu. is almost the same as that in a vacuum, the values of .epsilon. and .sigma. vary widely from those in a vacuum according to the composition ratios and other factors, as described above. If, in damp ground, the value of .epsilon. is about thirty times that in a vacuum and the conductivity is 10.sup.-3 S/m, the speed v of electromagnetic waves of frequency 1 GHz is determined from Equation (2) as 5.5.times.10.sup.7 m/sec, i.e., about one-fifth of that in the atmosphere.
When radar is employed for underground investigation, the substances and the composition ratios of the soil are usually unknown. Therefore, the speed of propagation of electromagnetic waves in the medium is unknown, and thus accurate information on the distance to a detected object can not be obtained using radar of a conventional type, although the presence of the object can be detected thereby. In the case of damp ground in which the speed of propagation of electromagnetic wave is 5.5.times.10.sup.7 m/sec, for instance, a computation of the distance to the detected object on the assumption that the speed of propagation of electromagnetic waves is the same as that in the atmosphere gives a false result which is about five times the true distance.
One conventional method of this kind is shown in "2.5 SPEED OF PROPAGATION IN UNDERGROUND AND MEASURING DISTANCE" of "UNDERGROUND RADAR SYSTEM", SANE 79-40, reported on Jan. 25, 1980, by Technical Report of IECE in Japan. In this report, a radar antenna must be located on the first place right over the buried object and the second place aparted from D meter from the first place using electromagnetic wave of the same one frequency signal, respectively.
In addition, it is also necessary to determine the speed of propagation v of electromagnetic waves when measuring the speed V of the detected object, since this speed V is expressed by: EQU .DELTA..epsilon.=-4.pi.V.multidot..omega./v (3)
where the angular frequency of the reception signal is .omega., the Doppler shift is .DELTA..omega., and the speed of propagation of electromagnetic waves is v (the negative sign in Equation (3) covers the case in which the object is moving away from the measuring system).
Accordingly, it is difficult to employ a conventional radar apparatus unmodified for determining the distance to an object and its speed of movement in a medium whose permittivity .epsilon., permeability .mu. and conductivity .sigma. are unknown.